Adult bone marrow erythropoiesis is primarily homeostatic, constantly producing new erythrocytes throughout adult life. However, during fetal development and in response to acute anemia in adults the situation is dramatically different. At these times stress erythropoiesis predominates, which rapidly produces large numbers of new erythrocytes. Little is known about the mechanisms that regulate stress erythropoiesis in humans. My laboratory utilizes a murine model for stress erythropoiesis and has demonstrated that BMP4 dependent signals are required for the rapid expansion of a specialized population of stress erythroid progenitors during the recovery from acute anemia. These progenitors have greater potential to rapidly generate large numbers of new erythrocytes than bone marrow steady state progenitors. In this proposal we will further characterize the BMP4 dependent stress erythropoiesis pathway by taking advantage of a robust experimental system that utilizes erythroid recovery during the period immediately following bone marrow transplant. Our preliminary data show that this recovery is mediated by erythroid short-term radioprotective cells, which utilize the BMP4 dependent stress erythropoiesis pathway. In the first aim, we will examine the mechanisms that regulate BMP4 expression in the spleen during the recovery from bone marrow transplant. In the second aim, we will examine the role of Hedgehog and BMP4 signaling in the specification and expansion of stress erythroid progenitors in the spleen. While in the third aim, we will examine the regulation of Scl, Gata2 and Gata1 by Hedgehog, BMP4 and hypoxia. These studies will provide key basic information concerning the regulation of stress erythropoiesis, which can be built upon to develop new therapies to treat anemia.